| As the sulfur content of crude oil increased significantly, the H2S corrosion has become an important problem in oil refining industry, especially in an atmospheric distillation tower overhead system. After desalting process, salts and sulphide compounds dissolved in crude can provoke the formation of a corrosion aqueous solution whose chemical composition involves the presence of both hydrogen sulfide (H2S) and hydrochloric acid (HCl). This corrosion system of H2S-HCl-H2O is very aggressive causing serious corrosion on metals during plant operation. Therefore, it is imperative to comprehensively study the corrosion behavior of these metals used in oil refining and to put forward the relevant protection technology.Many works have been performed related to the corrosion process of carbon steel and other metals in H2S-containing solutions. Most of these papers describe the influence of saturated H2S at atmospheric temperature on the corrosion phenomena. However, little research has been done on the corrosion behaviour and corrosion control technology of carbon steel in the corrosion system of H2S-HCl-H2O at the temperature of the condensation system in primary distillation plants, which usually is greater than 70℃. In the present work, the corrosion behaviour and mechanism of 20 carbon steel in the corrosion system of H2S-HCl-H2O at 90℃were systematically studied using weight loss method, electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The inhibitive effect of triethylene tetramine (TETA) as a neutralizer for the carbon steel in the corrosion system at 90℃was also investigated. The main conclusions were summarized as follows:1. The corrosion rate of 20 carbon steel increased significantly with the increase of H2S concentration at 90℃. The loose corrosion products without any protection property formed on the steel surfaces were composed of mackinawite. Severe corrosion cavities were observed on the carbon steel surface in the solutions containing H2S due to cementites stripped off from the grain boundary.2. The serious uniform corrosion was observed on the 20 carbon steel in HCl solutions at 90℃. The cathodic hydrogen evolution process at the metal surface was promoted with increasing H2S concentration, which contributed to the positive shift of the corrosion potential and the increase of the corrosion rates.3. The corrosion process of 20 carbon steel was controlled by the depolarization of H+ in cathodic process in H2S-containing solutions with different concentration of HCl at 90℃. The corrosion rate increased with increasing of HCl concentration. The non-Faradaic process became weak with the addition of HCl in the H2S solutions. As a result, the electrodes surfaces were greatly corroded while the corrosion cavities were not observed in the HCl-containing solutions. Additionally, the corrosion products, which consisted of mackinawite, were observed in the H2S-containing solution with or without HCl.4. The corrosion behaviour of 20 carbon steel was investigated in HCl solutions with different concentration of H2S at 90℃. Results showed that the corrosion process was mainly controlled by the depolarization of H+ in cathodic process when the HCl concentration was 0.005 wt%. The corrosion rate increased with the increase of H2S concentration. However, when the HCl was 0.100 wt.%, the corrosion process of carbon steel was controlled by a mixture of both the anodic iron dissolution and cathodic hydrogen evolution. Meanwhile, the addition of H2S could have an inhibition on the carbon steel corrosion under the conditions where HCl concentration was 0.100 wt.%.5. Potentiodynamic polarization studies suggested that TETA was a mixed type inhibitor, predominantly controlled anodic reaction. The maximum inhibition efficiency of TETA was found for the carbon steel in the 199.0 mg L-1 H2S+0.010 wt.% HCl solution of pH 6.05. The better inhibition effect was also observed in the sour solution with 341.7 mg L-1 H2S and 0.050 wt.% HCl. |
PDF Full Text Request